A common hydraulic servo steering system 1 of a motor vehicle such as a passenger car shown in FIG. 1 serves for the steering adjustment of the wheels 2, especially the front wheels, of a two-track motor vehicle. The wheels 2 are swivelled or turned about their steering axis by means of a steering gearing 3. This is initiated by the driver of the motor vehicle with his steering wheel 4, which is connected to the steering gearing 3 by means of a steering shaft 5, also called steering column, by interposing a so-called steering gear. The steering column 5 comprises an input shaft 6 and a pinion shaft 7. A supporting force can be applied to this steering gearing 3 by means of a so-called steering cylinder 8 formed as a hydraulic synchronized cylinder. This is done depending on the degree of left and right wheel turn imposed by the driver by means of a steering valve, the action of which is coupled to the torsion bar or the torsional moment applied to it, respectively, which in turn corresponds to the steering torque applied by the driver. Thus, the steering valve specifies the dependence between steering torque and supporting force.
Such customary hydraulic power steering systems or servo steering system, respectively, are characterized by high energy density, high maximum force as well as good dynamic performance.
The steering valve acts to set the supporting force by varying a steering valve opening, and thus acts in direct dependence on a steering torque applied to the steering wheel. Furthermore, steering systems are known, wherein the steering valve opening is set by means of a gearing, for example a pinion gearing assembly between the pinion shaft and the valve sleeve. In particular, such a gearing is provided in steering systems where, in addition, the ratio between the steering valve opening and the applied steering torque, i.e. the valve characteristics (also called the boost curve) can be varied by an actuator. Such steering systems are for example disclosed in DE 102007028529 A1, EP 1514766 A1 and DE 102004015991A1, and were developed under the term of “torque overlay” to extend the functionality of hydraulic steering in terms of automatic parking and lane keeping. While in conventional steering valves, the steering valve opening and thus the initiating steering support is directly associated to the steering torque applied to the steering wheel, herein additional adjustment is enabled by means of an actuator. For example, in a conventional steering system, the pinion shaft engaged in the rack is fixedly connected to the valve sleeve, while in valves according to the disclosure, for example, a relative movement caused by the actuator is possible. There are various technical approaches known to a person skilled in the art.
All these approaches suffer from the disadvantage that the gearing has play, and, as a consequence, in changes of steering direction there is a steering angle range in which the steering valve opening is not following the steering torque and is trailing. This strongly hampers steering control; and at best causing a very indirect steering control that is perceived as a disadvantage; generally, the driver will sense this play as an unsteady steering torque, especially with multiple changes of direction, for example in long lasting curves. This unsteady feeling is also called “handshake”. Such a handshake especially occurs at high differential pressures of both chambers in the hydraulic cylinder and with a steep boost curve, since in this working range, a small differential angle due to the play does cause clearly greater differential pressures.
Thus, a need exists to improve steering performance and in particular to avoid the disadvantages of play in the gearing that controls the steering valve adjustment, at least in special steering situations, and to improve the response performance of a hydraulic servo steering system.